The viewing of objects, including parts of the human anatomy, by the use of x-rays and other forms of penetrating radiation is known. In the case of x-rays, the radiation is directed at the object from one side, and the part of the radiation that penetrates the object is detected at the opposite side. An image may thus be obtained in which parts of the object that are more absorbent of x-rays, typically more dense parts of the object, appear as darker shadows, for example, by detecting the x-rays electronically and generating an image using a computer. In a computed tomography (CT) system, a series of x-ray images of a target are taken with the direction from the source to the detector differently oriented relative to the target. From these images, a three-dimensional representation of the density of x-ray absorbing material in the target may be reconstructed. Other methods of generating a three-dimensional dataset are known, or may be developed hereafter. From the three-dimensional data, a tomogram, which is a section in a desired plane, may be generated.
In a typical dental CT system, the patient sits upright, and the x-ray source and detector are mounted on opposite ends of a gantry that rotates about a vertical axis through the middle of the patient's head. In order to obtain sufficient data to cover the desired part of the head, which is most of the extent of the skull in the horizontal plane in CT imaging with high resolution, the detector must span a substantial distance in the circumferential direction perpendicular to the axis of rotation of the gantry, that is to say, the horizontal, direction.
Electronic detectors currently available include flat panel arrays of charge-coupled device (CCD) or other detectors, each of which converts incoming x-rays over a defined pixel area in a defined time to an electric charge that can easily be converted to a digital intensity value for subsequent computation. One flat panel detector commercially available from Varian Medical Systems, Inc., of Salt Lake City, Utah that is suitable for use in dental CT units has a pixel size of 127 μm (micrometers) square, and has an overall panel size of approximately 25 cm×20 cm. In dental imaging with a cone beam, because of the divergence of the beam towards the panel, that panel provides an effective Field of View approximately 16 cm×13 cm. When mounted with the long axis horizontal, the 25 cm length of the panel thus allows a Field of View with a diameter of approximately 16 cm, which is large enough to permit sufficient coverage of the imaged structures in the axial (horizontal) direction with high resolution for typical dental uses.
However, for most normal adults, the 20 cm height of the panel allows imaging only from the bottom of the lower jaw to about the bottoms of the orbits of the eyes (about 13 cm effective height at the level of the object being viewed). That is sufficient for most dental and oral surgery applications, but for some classes of orthodontic and orthognathic surgery applications an x-ray image up to the level of the glabella, roughly the level of the eyebrows, is essential. Such images, known as “full face” have in the past been produced by conducting two overlapping scans of 13 cm height at different levels and merging the images. Conducting two scans increases the radiation dose to the patient. Merging the images seamlessly is difficult, especially as the time taken to reposition the gantry, or the patient, between the two scans allows the patient to move. It would be possible to use a 25 cm square detector panel, which would have both the width to produce full coverage CT scans of the mouth diametrally, and the height to produce full-face scans in a single scan for about 98% of human adults, but the cost of detector panels increases disproportionately to the size of the panel, and could not easily be justified, when the full height is seldom required.